FIG. 1 is a block diagram showing a conventional radio paging system. To page radio paging receiver 106-1 or 106-2 from telephone terminal 101, the number of radio paging receiver 106-1 or 106-2, and the message if there is one, are input from telephone terminal 101. This information is then input to central station 103 via public switched telephone network (PSTN) 102 as a paging request signal. At a central station 103 this paging request signal is converted to a paging signal sequence and transmitted to a plurality (herein denoted by n) of base stations 1041 to 104-n. Base stations 104-1 to 104-n receive this paging signal sequence using line terminators 107, convert it to a radio signal (a radio wave) in transmitter 108, and transmit it simultaneously at the same frequency F.sub.f to radio zones 105-1 to 105-n of each respective base station. Radio paging receivers 106-1 and 106-2 each receive the radio signal at the predetermined frequency F.sub.f, and when either of these radio paging receivers detects a paging signal addressed to itself, it notifies the user that he or she has been paged.
FIG. 2 shows an example of the configuration of a paging signal sequence received by a radio paging receiver. This paging signal sequence comprises a frame sequence of period T comprising m frames of signal length .alpha. joined together. The i-th frame comprises a synchronization signal and k.sub.i paging signals. Each paging signal comprises an address signal and a message signal. The receive function of a radio paging receiver is activated only during the receiving period of one or more pre-allocated frames of the m frames, whereupon it receives the frame or frames in question. This is called "intermittent receiving" and is intended to extend battery life in a radio paging receiver. For example, on the assumption, that radio paging receivers 106-1 and 106-2 have been allocated to the same frame i, the receive function of these radio paging receivers is activated only in frame i whereupon they receive paging signals in this frame i. Further, if both radio paging receivers 106-1 and 106-2 are paged in the same frame, then for example the paging signal transmitted to radio paging receiver 106-1 is paging signal i-l having the address signal of this receiver, and the paging signal transmitted to radio paging receiver 106-2 is paging signal i-2 having the address signal of that receiver. When a radio paging receiver has detected a paging signal addressed to itself within the received frame, it outputs an audible tone or other alert, thereby notifying the user that he or she has been paged.
A problem with the radio paging system described above is that because it is a one-way communication system, the caller cannot know whether or not his paging request has reached the desired radio paging receiver. To overcome this problem, a two-way radio paging system has been proposed, wherein a radio pager with a transmission function is, used instead of a receive-only radio paging receiver, so that when a radio pager has detected a paging signal addressed to itself, it transmits a response signal to the base station to the effect that a paging signal has been received.
FIG. 3 is a block diagram showing a conventional two-way radio paging system. In this system, instead of central station 103, base stations 104-1 to 104-n and radio paging receivers 106-1 and 106-2 shown in FIG. 1, there are used central station 111, base stations 112-1 to 112-n and radio pagers 113-1 and 113-2, all of which are capable of two-way communication.
To page radio pager 113-1 or 113-2 from telephone terminal 101, a paging signal sequence is transmitted simultaneously at the same frequency F.sub.f to radio zones 105-1 to 105-n of each respective base station, in the same manner as the prior art example illustrated in FIG. 1. Radio pagers 113-1 and 113-2 each receive this paging signal sequence, and when either of these radio pagers detects a paging signal addressed to itself, it notifies the user that he or she has been paged. Subsequently, the paged radio pager 113-1 or 113-2 transmits to a base station a response signal to the effect that a paging signal has been received. It is also possible at this point to add a short message signal. The base station of the radio zone in which the radio pager is located receives this response signal at receiver 114 (the response signal transmitted by radio pager 113-1 is received by base station 112-1 and the response signal transmitted by radio pager 113-2 is received by base station 112-2), and transmits the response signal via line terminator 107 to central station 111. Central station 111 can then report via telephone network 102 to telephone terminal 101 that paging has been successful.
FIG. 4 shows an example of the configuration of a response signal transmitted by a radio pager. This response signal comprises a synchronization signal, the radio pager ID, and the response information. The response information can include a short message in, addition to a response to the effect that a paging signal has been received.
It is assumed herein that as in the case of a mobile phone, the frequency of the backward signal which a radio pager uses to transmit its response signal is set uniquely in relation to the frequency of the forward signal used to transmit the paging signal. Namely, F.sub.b is assumed to be set uniquely in relation to F.sub.f. Now, in a radio paging system a single forward signal frequency is used in all the radio zones. Therefore when a plurality of paging signals are transmitted in the forward signal, in all the radio zones a plurality of radio pagers will respond simultaneously using the same backward signal frequency. To avoid collision of such response signals and to improve the transmission quality of the backward signal, at least those radio pagers which have been paged simultaneously must send back their response signals at different times. For example, the backward signal is divided into a plurality of slots, each slot is allocated to one of the radio pagers which have been paged simultaneously, and the response signal from each radio pager is inserted in its allocated slot.
FIG. 5 and FIG. 6 are block diagrams showing two examples of the configuration of a central station. FIG. 5 shows an example of the configuration of central station 103 used in the one-way radio paging system illustrated in FIG. 1, while FIG. 6 shows an example of the configuration of central station 111 used in the two-way radio paging system illustrated in FIG. 3.
Central station 103 used in the one-way radio paging system comprises paging switch 121, encoding unit 123, distributor 124, and a plurality of line terminators 126. Paging switch 121 is connected to the telephone network and outputs paging request signals from this telephone network to encoding unit 123. Encoding unit 123 converts these paging request signals to a paging signal sequence. Distributor 124 distributes this paging; signal sequence to each base station. Line terminators 126 each terminate a line to a base station and transmit the paging signal sequence from distributor 124 to a base station.
As opposed to this, central station 111 used in the two-way radio paging system comprises paging switch 122, encoding unit 123, distributor and concentrator 125, and a plurality of line terminators 127. Paging switch 122 is capable of two-way operation, while encoding unit 123 is on a par with the encoding unit of central station 103 shown in FIG. 5. Distributor and concentrator 125 distributes the paging signal sequence output by encoding unit 123 to each base station and also concentrates the response signals received by line terminators 127 and sends back the response information to the paging request sources via paging switch 122. Line terminators 127 terminate the lines to the respective base stations, transmit the paging signal sequences from distributor and concentrator 125 to the base stations, receive response signals coming from the radio pagers via the base stations, and input these response signals to distributor and concentrator 125.
If a backward signal is divided into a plurality of slots for allocation to the radio pagers, an increase in the number of radio pagers transmitting response signals will necessitate an increase in the number of slots. Moreover, an increase in the length of the response signal from each radio pager will necessitate an increase in the number of bits comprising a slot. In either case, this will require an increase in the transmission rate and capacity of the backward signal.
However, a higher backward signal transmission rate means that the received power required at a base station becomes larger and the output power of a radio pager has to be increased. This results in increased power consumption at a radio pager and shortened battery life.
Although varying the backward signal frequency for each radio pager could be considered, this would result in poor frequency utilization and in complicated control of radio pagers.
It is an object of the present invention to provide a radio paging system which overcomes this sort of problem and which, even when the number of radio pagers is increased, is capable of transmitting a response signal from each radio pager at a low power level and of making effective utilization of frequency.